Soot definition, formation, influence for a boiler and soot removal by soot blower

2019-04-10

When we say “soot” in boiler, what are we talking about exactly? Following is my understanding for related terms.

Soot: fly ash that contains significant amount of carbon (should be avoided in high efficiency combustion system).

Fouling: fly ash settled on heat exchanger tubes and walls.

Fly ash: ash leaving the combustion system together with flue gas.

Slag: ash that has undergone a high temperature and is (or was) fully or partly molten (liquid).

In my understanding I get the following:

Fly-ash : ash leaving the combustion system together the flue gas

Fouling : fly-ash settled on tubes and walls

Soot : fly-ash that contains significant amount of carbon (should be avoided in high efficiency combustion system)

Slag : ash that has undergone a high temperature and is (or was) fully or partly molten (liquid)

Slag normally is very sticky (like glue) and difficult to remove. For typical fuels, the gas temperature in furnace, empty pass and superheater can be high enough to find slag.

In economizer or air preheater gas temperature is significantly lower than the melting temperature of fly ash. It settles like dust. Dust is easy to remove by various cleaning methods.

Certainly, there is fly ash from some combustion process that is very sticky even being not molten. For example, soot from boiler fired by biomass is very sticky. For waste heat boiler in non-ferrous metal metallurgy industry, soot is also very sticky and high content.

clean tubeclean tube

dirty tubedirty tube

In any combustion process of fossil/biomass fuel, there will be always some unburned carbon (soot) generated and some ash carried with flue gas. Soot is the inevitable result of burning. Molten ash settled down on tube surface is called slag, loose ash settled down on tube surface is called soot.

A little bit of soot is normal, but excessive soot on tubes can greatly inhibit heat transfer. Accumulated soot deposits have thermal conductivity of only 0.0581-0.116w/m2*℃, while boiler tubes have 46.5-58.1w/m2*℃. Heat transfer loss in a dirty boiler rises dramatically. The result is lower boiler efficiency, higher flue gas temperature, more fuel consumption, less steam output.

So regular cleaning for boiler tubes to avoid soot buildup is very important. Up to now, different soot cleaning methods have been utilized. Rapping system, steam soot blower, water cannon, sonic horn, shock wave soot blower, pneumatic pulse soot blower are tried in different cases.

We can provide effective soot blower for travelling grate boiler, CFBC boiler and SCR, ESP, etc.

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